1. Field of the Invention
The present invention relates to a projecting optical system which projects an image indicating element.
2. Description of Related Art
For example, Japanese Unexamined Patent Publication No. 2-79037 discloses a projecting optical system including a first projection lens system which forms an intermediate image of an image indicating element and a second projection lens system which projects the intermediate image onto a screen. The first and second projection lens systems have optical axes intersecting at a predetermined angle. In an optical arrangement disclosed in JPP 2-79037, an extension (i.e., plane) of a surface of an image indicating element of the first projection lens system, a plane passing through a principal point of the first projection lens and perpendicular to the optical axis thereof, and an extension (i.e., plane) of the intermediate image intersect on one straight line. In another arrangement, an extension (i.e., plane) of a screen of the second projection lens system, a plane passing through a principal point of the second projection lens and perpendicular to the optical axis thereof, and an extension (i.e., plane) of the intermediate image intersect on one straight line. With these arrangements, a projecting optical system having no trapezoidal distortion can be realized.
In the projection system proposed in JPP '037, an optical element having a positive power and a prism function is provided at a position in which the intermediate image is formed. The optical element functions as a pupil coupling element to couple pupils of the first and second projecting optical systems. Due to the prism function, the bundle of rays transmitted through the first projection lens system is bent towards the optical axis of the second projection lens system. Due to the positive power, the bundle of rays transmitted through the first projection lens system is converged onto the pupil of the second projection lens system. This allows for the miniaturization of the projection lens of the second projection lens system.
It has been proposed to use a decentered (i.e., eccentric) Fresnel optical element, such as a decentered Fresnel lens or mirror as the positive power optical element having a prism function. However, the decentered Fresnel optical element causes Fresnel fringes which are projected onto the screen, resulting in a dimmed projection image.
If a transmission type screen with a structural periodicity having a lenticular lens and a Fresnel lens is used, a moire effect would be caused by the Fresnel fringes of the decentered Fresnel optical element and the Fresnel fringes of the transmission type screen or fringes of the lenticular lens. Similarly, if the image indicating element is comprised of a liquid crystal panel having pixels in a matrix arrangement, a moire effect would be caused due to the periodicity thereof and the periodicity of the Fresnel optical element. As is well known, the image in the image plane in which the moire effect appears is indistinct.